The human vision system has evolved based on an environment for processing information that exists only in nature under continuous full spectrum ambient lighting conditions. Relatively recently, during the past 400 years, we have exposed our vision systems to unusual new requirements. The reading of printed text and pictures in artificial light, photographs, television and computer displays, all have colors restricted fundamentally to three to five color primaries.
Most film, video, digital cameras and display systems are based upon the three-color, metameric theory of chromatic capture, using red-green-blue or cyan-magenta-yellow primaries. Hence, the color range or gamut is limited by the selected color primaries supported by the display systems. In addition, the spectral range of conventional systems is constrained to these primaries rather than capturing the full radiant spectral signature of a scene. The full spectrum is necessary to replicate what the human vision system is capable of seeing. The full spectrum is also required for further analysis for scientific and surveillance applications.
Metameric or combinatory systems are based on a mathematical representation whereby it can be shown that any color that can be sensed by a human may be created by combining only three unique, suitably spaced spectral primaries. However, the inverse is not true: three unique, fixed primaries cannot create all colors. In order to cover the entire spectrum of human sense, primaries either have to be expanded to more than three or shifted to cover the spectra being replicated. Yet even with expanded primary systems, the bandwidth of the primaries and their consequent metamer are critical parameters, and cannot accurately represent the continuous spectral functions or chromatic separation that a normal human is capable of perceiving. Where illumination is non-continuous, such as a fluorescent lamp, its band or line spectra further complicates metameric chromatic capture and replication. Four or five primaries are sometimes used for ink on paper or special displays, thus expanding the gamut somewhat, but still cannot reproduce all of the colors that humans can see.
Furthermore, metameric models cannot represent the full radiant spectral energy in a scene. The metameric representation of color is only a psychophysical phenomenon dependent on human perceptual processes. This conventional technique is not a physical representation of the reflective or luminescent spectra for the purposes of spectral analysis. The use of metameric primaries for chromatic representation also excludes valuable data located in the invisible regions of the spectrum, including X-rays, the ultraviolet and infrared.
A conventional display system based on combining filtered colors cannot perfectly replicate what a human can see in the field, where the human visual system is capable of sensing a spectrum that cannot be physically displayed by select primary light sources. Furthermore, such conventional chromatic display systems are inflexible, since they are based on a mythical “standard” human observer, and therefore cannot be readily adjusted for human perceptual diversity or handicaps. Essentially, all tri-stimulus display systems based on optically filtered primaries are a form of data compression accomplished by discarding portions of the spectrum.
Recent biological research confirms that the perception of the full spectrum and full luminance in a scene, at high spatial and temporal resolutions, is minimally necessary for correctly replicating images with subtle chromatic nuances such as transparency, luminescence, mirror-reflectance and texture. It is also important to replicate the full continuous spectra of a scene to overcome numerous chromatic and other visual illusions and to achieve color constancy despite chromatic shifts in ambient lighting.